The composition and general mode of operation of an ink jet printhead, for instance one according to the top shooter type thermal technology, i.e. that emits ink droplets in a direction perpendicular to an ejection module, are widely known in the sector art and will not therefore be described in detail here.
Ink jet heads are commonly used in producing serial printers in which the nozzles are arranged perpendicular to the line of print and the head is moved transversally over the surface to be printed.
The ejector units are obtained as chips from a semiconductor substrate, typically a silicon wafer, with processing technologies similar to those employed for the production of integrated and/or hybrid circuits.
In short, various layers are deposited on a face of the substrate to make up the ejection resistors and the active electronic components, and a layer of photopolymer. Using photolithographic techniques, the ejection cells and ink delivery channels are made in the photopolymer and an orifice plate provided with ejection nozzles built in correspondence with the cells is mounted.
Today's technology tends to produce ever larger numbers of nozzles per head, and ever higher print definitions with high working frequency and produce ever smaller ink droplets. This requires actuators of reduced dimensions, very short hydraulic circuits and channels, high levels of precision in positioning and assembling the components, while also accentuating the problems of the differing coefficients of thermal expansion of the materials making up the head.
High reliability is also required of the printheads, especially when there is to be interchangeability of the ink tank. These heads, called semifixed refill heads, have in fact an effective life close to the life of the printers.
Thus there is a need to develop and produce fully integrated, monolithic heads, in which the ink channels, the selection microelectronics, the resistors and the nozzles are integrated in the wafer.
The latest heads for serial printing have a special nozzle disposition along an edge of the ejection module, they use simplified feeds for the ink through a distribution slot or channel in the unit, common to all the cells and, in some cases, have the orifice plate integrated in the unit. During manufacture, a sacrificial layer of photopolymer that is subsequently eliminated is used in making the cells and delivery channels, and a structural layer for formation of the nozzles.
Serial type printers are moreover somewhat cumbersome and, therefore, unsuitable for use with portable and/or compact equipment.
Ink jet heads that can be used in parallel or serial-parallel printers are known. The line of a page is printed in a single stroke without any need for a scanning movement across the surface being printed, or with a scanning that is limited in relation to the longitudinal movement of the page.
Heads for parallel or serial-parallel type printers are generally manufactured with various ejector modules set side by side. It is in fact difficult to produce—with an acceptable yield—large-size chips or single units that are defect-free and can define all the nozzles in the parallel printing area. In addition, the heads in a single unit could not draw advantage from the ink feed simplifications of today's serial heads, due to the weakening that would be caused by a large-size slot in the unit.
Ejector modules for parallel printers are of limited dimensions (½″, 1″) and are assembled on a common support in such a way as to obtain an aligned disposition of the nozzles like in a single unit. However other problems arise when this structure is chosen, such as, for example, that of the difficulty in setting integrated units side by side, due to presence of the ink delivery slots.
Recently, ink jet heads have been developed for serial printing with numerous nozzles extending over a consistent part of the ejection module and suitable for simultaneously printing a large number of dots along the printing area and/or on various printing lines. These extensive heads are also mechanically weak, are complex to manufacture and many of the structural problems remain unresolved.